Variable speed screw gearing mechanism



July 23. 196s l W. J. ROANTREE 3,393,574

VARIABLE SPEED SCREW GEARING MECHANISM Filed April 2l., 1967 4Sheets-Sheet l m l WML/,4M JT oA/vmef 1 cg P- w INVENTOR..

July 23, 1968 w. J. ROANTREE VARIABLE SPEED SCREW GEARING MECHANISM 4Sheets-Sheet 2 Filed April 2l, 1967 W/LUAM J. RWM/7R55 INVENTOR,

WQ/J 9%@ ATTORNEY July 23, 1968 W. .1. ROANTREE `VARIBLE SPEED SCREWGEARING MECHANISM Filed April 2l, 1967 4 Sheets-Sheet W/L/AM JRoA/vrRE-E INVENTOR.

ATTOR/VEX July 23, 1968 w. J. ROANTREE VARIABLE SPEED SCREW GEARINGMECHANISM 4 Sheets-Sheet 4 Filed April 21. 1967 m. .Al-ll m .n .www

WML/AM J'. RUAA/TREE INVENTOR.

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ATra/MusxA United States Patent O i v 3,393,574 y VARIABLE SPEED SCREWvGEARING MECHANISM William J. Roantree, Port Washington, N.Y., assignorto Roantree Electro-Mech Corporation, Port Washington,

N.Y., a corporation of New York Continuation-impart of application Ser.No. 614,339,`

Feb. 6, 1967. This application Apr. 21, 1967, Ser. No. 632,769

12 Claims. (Cl. 74-424.7)

where:

LL=lead of the lead screw (positive for right-hand thread) LR-:lead Aofdifferentialv roller (positive if right-hand thread) l Y DL=diameter ofthe driving surface of the lead screw if male thread if female thread),and y DR=diameter of the driving surface of the differential roller (lif male thread, if `female thread).

Variations in thevalue of L may be effectedby changing one or bothof DLand DR.

This application is a fcontinuation-in-part of application-Ser. No.614,339, filed Feb. 6,zl967, entitled, .Gearing Mechanism. Y

l .Background oftheA invention I (l) Field of theinvention-Thisinvention relates to a variable gearing mechanism whichconverts rotaryfmotion to linear motion. The mechanism includes va leadscrew and one or more differential rollers having screw threads ofgenerally the same pitchv as those4 on the lead screw and drivinglyconnected to such threads. As the lead screw rotates, the differentialroller is rotated thereby, the resultant speed of the carriage uponwhich the roller is lmounted being determined by the ratio of theeffective diameters of the lead screw and roller at their zones ofdriving engagement.

(2) Description of the prior art.Conversion of rotary motion to linearmotion has been secured bythe use of a lead screw having two threadportions of the same hand but somewhat different pitch threadedlyengaged with respective nuts. Rotation ofthe lead screw causes the nutsto separate or approach each other, depending upon the direction ofrotation of the screw, at arate which is a function 'of the differencebetween the pitches of the respective thread portions. Suchv priormechanism may be employed to give large mechanical addvantages inreciprocating mechanisms such as platen presses, lifting devices, jacks,and the like. Such prior mechanism is of fixed ratio, however, sincewith a given difference in pitch of the thread portions the linearmotion of the two nuts relative to each other with respect to theangular motion of the screw is fixed.

Patented July 23, 1968 ice Summary of the invention The gearingmechanism in accordance with the invention provides a simple means forconverging rotary motion to linear motion, the ratio between the rotarymotion and the linear motion being variable but substantiallysynchronized at any selected adjusted position of the mechanism. In some'embodiments of the mechanism in accordance with the invention themechanism is manually adjustable so as to vary the ratio between rotarymotion and linear motion as desired. In other embodiments of themechanism the rate of linear motion is automatically adjusted relativeto the rate of rotary or angular motion as by changing the effectivediameter of the driving portion ofthe lead 'screw at one or more zonesthereof relative to the effective diameter of thedriving portion of thedifferential roller or rollers drivingly engaged with such screw. Inother embodiments the effective diameter of the driving portion of thethread on the lead screw is constant and the effective diameter of thedriving portion of the thread on the differential roller or rollers isautomatically varied, as by being made responsive to changes in the loador thrust imposed upon the differential roller. This may be accomplishedby making the differential roller with the driving portion of the threadthereon of varying diameter and mounting such roller so that theinclination of its axis may vary in accordance with the thrust imposedthereon, whereby selectively to present thread portions of differentdiameter for driving engagement with the thread on the lead screw.

In the various embodiments of variable gearing mechanism disclosedherein the threads on the lead screw and the differential roller orrollers are of generally the same pitch and opposite hand, the threadson the lead screw and the differential roller being in meshing, directdriving engagement. In certain disclosed embodiments the radially outersurfaces of the lands on one of the members drivingly engage the rootsof the grooves of the screw thread on the other of said members. Inother disclosed embodiments, radially outer edges of the lands on one ofsaid members drivingly engage corresponding sides or flanks of the landson the other of said members.

In those embodiments wherein the ratio between the speed of rotarymotion and the speed of linear motion is' automatically varied inaccordance with the thrust imposed upon the differential roller as ittravels linearly, such speed of linear motion decreases as the thrustincreases. This precludes the overloading of the power source. Suchmechanism is of advantage in applications such as platen presses whereinit is desired that the platen shall travel more slowly and with anincreased thrust at a certain portion or portions of its working strokesuch as at the end thereof.

Brief description of the drawing FIG. 1 is a somewhat schematicfragmentary view partially in vertical axial section and partially inside elevation of a first embodiment of variable gearing mechanism inaccordance with the invention;

FIG. 2 is an enlarged fragmentary view of a portion of the mechanism ofFIG. l showing on an enlarged scale -the zone of driving engagementbetween the flanks of the shown with the differential rollers in thepositions which they assume when they are axially unloaded, the sectionbeing taken along the line 3 3 in FIG. 5 looking in the directions ofthe arrows;

FIG. 4 is a fragmentary view of the mechanism of FIG. 3 shown similarlyto the view in FIG. 3 but with the differential rollers in the positionswhich they assume when they are axially loaded;

FIG. 5 is a View in transverse section of the gearing mechanism VofFIGS. 3 and `4 with the rollers in the loaded position of FIG. 4, thesection being taken along the line 5*-5 of FIG. 4 looking in thedirection of the arrows;

FIG. 6 is a fragmentary somewhat schematic view partially in verticalaxial section and partially in side elevation of a third embodiment ofvariable gearing mechanism in accordance with the invention, suchmechanism, which is variable in response to variations in the thrustimposed thereon and is here shown as being axially unloaded, having alead screw with axially spaced portions of different effective drivingdiameters and differential `rollers-drivingly engaged therewith, suchrollers having zones of different effective driving diameters and beingso mounted that their inclinations relative to the axis of the leadscrew automatically change whereby to maintain drivingly relationshipwith the lead screw as the rollers travel therealong, the section beingtaken along the line 6-6 of FIG. 7;

FIG. 7 is a view in vertical transverse section through the mechanism ofFIG. 6, the section being taken along the line 7-7 of FIG. 6 looking inthe driection of the arrows;

FIG. 8 is a fragmentary somewhat schematic view partially in verticalaxial section and partially in side elevation of a fourth embodiment ofgearing mechanism in accordance with the invention, the mechanism beingshown in substantially axially unloaded condition, such gearingmechanism employing a lead screw having a thread of constant effectivedriving diameter and tiltable differential rollers having threads ofvarying diameter in driving engagement with the thread on the leadscrew, the angle of inclination of the differential rollers relative tothe lead screw being variable in accordance with variations in the endthrust imposed upon the rollers, the section being taken along the line8 8 of FIG. 9 looking in the direction of the arrows; and

FIG. 9 is a view in vertical transverse section through the mechanism ofFIG. 8, the section being taken along the line 9-9 of FIG. 8 looking inthe direction of the arrows.

Description of the preferred embodiments As evident from the above,there are shown and described herein four embodiments of variablegearing mechanism in accordance with the invention. Such embodiments areas follows:

(l) FIGS. 1 and 2;

(2) FIGS. 3, 4, and 5; (3) FIGS. 6 and 7; and (4) FIGS. 8 and '9.

In embodiment (1) the ratio between the speed of rotation of the leadscrew and the linear travel of the carriage bearing the differentialroller is manually adjustable. In embodiments (2), (3), and (4) theadjustment of such ratio is automatic, being responsive to variations inend thrust imposed upon such carriage and thus upon the differentialrollers mounted thereon. In all of the disclosed mechanisms thevariation in the ratio DL/DR effects a change in the rate of travel ofthe differential rollers along the lead screw in accordance with theabove equation In all of embodiments (l), (2), (3), and (4) the factorDR (effective driving diameter of the differential roller) is varied; inembodiment (3) the factor DL (effective driving diameter of the leadscrew) is also varied.

Turning now to the drawings, the gearing mechanism of FIGS. l and 2 isdesignated generally by the reference character 10. In such mechanismthere is a longitudinally extending lead screw 11 which has Unthreadedstub shaft or trunnion portions 12 and 13 on the ends thereof. Suchtrunnions are mounted in bearings 14 in spaced upstanding supports 15which extend in spaced parallel relationship from a base 16. The base 16and the supporting portions 15 are in turn mounted upon a xed structuresuch as a frame 18. The lead screw 11 is rotated by driving means (notshown) connected to a portion 17 of the trunnion 12 extending beyond itssupport 15.

In the embodiment shown, a single differential roller 20 is mountedbelow the lead screw 11 with the axis of the roller parallel to that ofthe lead screw. The differential roller, which is threaded in its main,intermediate extent, has stub shafts or trunnions 21 and 22 on oppositeends thereof, such trunnions being mounted in bearings 24 in respectiveupstanding parallel supports 25. Supports 25 are integrally connected toa base`26, the supports 25 and the base 26 constituting a carriage andbeing mounted for movement with the roller 20 axially of the lead screw11 in a manner to be described. The differential roller 20 has threadsthereon of the same pitch as but opposite hand from the threads on thelead screw 11. In the embodiment shown, the threads on the lead screwand on the differential roller are of a modified Acme type, the axialwidth of the lands of the thread on the differential roller beingsomewhat less than the axial width of the lands on the lead screw.

Adjustment of the distance between the axes of the lead screw and thedifferential roller, while permitting the described axial movement ofthe differential roller and its carriage, is provided by disposing aplate or table 27 having its upper surface T-slotted at 43 between thesupports 15, the lower surface of the base 26 of the car* riage beingprovided with T-shaped projections slidably received in the T slots 43in the upper surface of the plate 27. Plate 27 is selectively adjustedtoward and away from the axis of the lead screw 11 by adjusting screws29 which are threadedly received in spaced vertical passage 30 in theportion 16 of the support for the lead screw. As the plate 27 is thusadjusted it carries with it the carriage 25, 26 and thus thedifferential roller 20, whereby to adjust the distance between the axesof the lead screw and the differential roller. This results in a changeof the ratio DL/DR in the above equation in a manner to be described.

The carriage 25, 26 is shown attached to a thrust rod 31 which extendsthrough an opening 32 in the left-hand support 15. The thrust rod 31 maythus reciprocate relative to the support 15. The differential roller,carriage 25, 26, and the thrust rod 31 will (l) remain axially fixed orwill travel either to the right (2) or the left (3) with a givendirection of rotation of the lead screw 11, depending upon the value `ofDL/DR. `Such first condition (l) is obtained when the position of thecarriage 25, 26 relative to the lead screw is adjusted so that DL=DR;such carriage travels in one direction, for example direction (2), whenDL is greater than DR and travels in the other direction, for exampledirection (3), when DL is less than DR.

As will be seen more particularly in FIG. 2, the lead screw has lands 34having axially short circular cylindrical radially outer surfaces 35 andgrooves 38 between the land 34. The grooves 38 have roots 36. The lands34 have a left ank 37 and a right flank 39, the axial distance betweenflanks 37 and 39 being such throughout a substantial part of the radialdepth of the grooves 38 as freely to receive the lands 40 of the threadon the differential roller 20. Lands 40 similarly have axially shortcircular cylindrical outer surfaces 41 and grooves 46 between successivelands 40. Each of grooves 46 has a root 42 and a right and a left flank45 and 44, respectivel Diiving engagement between the threads on thelead screw and the differential roller is effected in this embodimentbetween the right flank 39 of the lands 34 on the lead screw and theleft-hand radially outer edge 47 of the lands 40 of the thread on thedifferential roller, such points of driving engagement being designated49.

The configuration of the screw threads are such as to maintain freedomfrom engagement between the lands 34 on the lead screw and lands 40 onthe differential roller other than at the point 49 throughout anextended range of adjustment of the distance between the axes of suchmembers. For this purpose the threads on the members are made such thatthe angle a between the ank 39 and the transverse plane of the leadscrew somewhat exceeds the angle between the flank 44 and a similarplane of the differential roller. It will be obvious that the distancesRL and RR from the point of driving engagement 49 to the axis of thelead screw 11 and differential roller 20, respectively, are 1/2 DL andDR, respectively.

The embodiment of gearing mechanism of FIGS. 3, 4, and 5 is generallydesignated by the reference character 52. In such embodiment there is alead screw 54 which is engaged by a plurality of differential rollers 55(four shown). Lead screw 54 is rotatably mounted in suitable bearings inspaced parallel upstanding supporting members 56 and 57 which areintegrally secured to a horizontal base 59. The base 59, in turn, isfixedly secured to a fixed member 60 which may be a frame of a machine.The mechanism 54 is shown reciprocably driving a platen 61 through themedium of thrust rods 62 which are secured to the carriage in which thedifferential rollers 55 are mounted. The thrust rods 62 are slidinglyreceived in passages 64 in the support 56 so that both the platen andthe carriage are accurately guided for linear motion and are preventedfrom rotation about the axis of the lead screw 54.

The carriage 65 which carries the differential rollers 55 includes acircular cylindrical .shell 66 which is disposed coaxially of the leadscrew and telescoped thereabout. The opposite ends of the shell 66 areof reduced radial thickness, there `being a transverse annular innershoulder 70 at the juncture of the intermediate main portion of theshell and such end portions thereof. Within the thus formed seats at theopposite ends of the shell 66 there are disposed annular inner plates 67and, outwardly thereof, outer or end closure plates 69. The plates 67and 69 are retained in their respective seats by end portions 71 of thethin outer portions of the shell, such end portions being bent inwardlyover and tightly gripping the radially outer edges of the plates 69.

The plates 67 and 69 have central openings 72 therethrough whichSlidingly and guidingly receive the radially outer portions of thethread 54 on the lead screw. Each of the differential rollers 55 isprovided with a pintle or stub shaft 74 on each end thereof, the pintles74 on each roller being slidably received within radially extendingoppositely disposed axially aligned slots 75 in the respective innerplates 67. Such construction permits the differential rollers 55 totravel from the position thereof shown in FIG. 3, wherein the carriageand the rollers are axially unloaded and lie with the pintles 74adjacent the inner ends of the slots 75, to the position shown in FIG. 4wherein the carriage and rollers are substantially axially loaded andthe rollers lie in their radially outward position with the pintles 74at the outer end of the slot 75. The rollers 55 are constantly urgedradially inwardly toward the position of FIG. 3 by resilient belts 77which are entrained over pulleys 76 affixed to each of the differentialrollers 55 beyond the ends of the screw thread thereon.

The lead screw 54 has a screw thread of generally buttress orsaw-toothed configuration in section, the successive lands 79 of suchscrew threads having radially outer edges 80, the grooves 84 Ibetweensuccessive lands 79 having a V-shaped root 81. The inclination of therear or left fiank 82 of the lands 79 with respect to a transverse planeis substantially greater than that of the forward or right flank 85 ofsuch lands. The lands 79 on the lead screw have a direct meshing anddriving engagement with the lands 86 on the differential rollers 55. Inthe embodiment shown the lands 86 are generally complementary to thegrooves 84 between lands 79 on the lead screw with the exception thatthe right or forward flank 90 of lands 86 and the left or rear flank 91thereof are inclined relative to a plane transversely to the axis of theroller at an angle which is somewhat less than the angles of inclinationon the flanks 82 and 85 of the lands 79 on the lead screw. As aconsequence, driving engagement between the lead screw and thedifferential rollers takes place between the outer edges 87 of the lands86 of the differential rollers and the rear or left-hand flank 82 of thelands 79 of the lead screw. When the differential rollers lie in theirunloaded position of FIG. 3, edges 87 of their lands substantiallyengage the roots 81 of the thread on the lead screw, there beingclearance between the edges 80 of the lands on the lead screw and theroots 89 of the threads on the differential rollers.

It will be apparent that when the differential rollers lie in theposition of FIG. 3, the effective diameter DR of the driving portion ofthe rollers is equal to its diameter and the effective diameter DL ofthe driving portion of the lead screw has its smallest value.Accordingly, the value of the lead L yielded by the mechanism is at amaximum. When the carriage and thus the differential rollers aresubjected to appreciable forwardly directed thrust, that is, there issubstantial opposition to the travel of the platen 61 to the left, thedifferential rollers 55 are thrust radially outwardly to a positionwhich is afunction of the value of such opposing thrust. FIG. 4 shows aposition of maximum thrust with the differential rollers lying in theiroutermost terminal positions.

It will be seen that as the differential rollers are progressivelythrust radially outwardly from the position of FIG. 3, the value of DLprogressively increases; the value of DR remains constant, since thedrive to the differential rollers always takes place by way of the edges87 of their lands 86. Consequently, as the opposing thrust upon theplaten 61 increases, the value of L, that is, the speed of linear travelof the carriage and the differential rollers, progressively decreases.As pointed out above, this is a valuable featuer, since it protects theprime mover against overload, and yields a desirable increase in theforce which the platen 61 can exert in that portion of a working strokethereof in which opposition to the travel of the platen increases.

The third species of gearing mechanism, shown in FIGS. 6 and 7, isdesignated generally by the reference character 100. Such gearingmechanism is shown as driving a platen 101 through the medium of thrustrods 102 which are secured at one end to the platen and at the other toa carrier or cage 104 mounting and enclosing differential rollers 113forming a part of the geairng mechanism, said rollers 113 drivinglycooperating with a lead screw 105. The lead screw 105 is shown supportedat its left-hand end in a supporting means and at its right-hand end ina supporting means 110', both of which may be portions of a fixed frame,there being suitable bearings 111 disposed between supports 110, 110 andthe lead screw. The lead screw may be driven by a means (not shown)drivingly connected to the portion 112 of the lead screw projectingbeyond the support 110. Thrust rods tyare guidingly received in bores insupporting means T he differential rollers 113 cooperate with a leadscrew in a manner generally similar to that of the embodiments of FIGS.1-4, inclusive, of applicants prior application Ser. No. 614,339. In thepresent embodiment, however, the lead screw 105 and differential rollers113 cooperate so that with a given speed of rotation of the lead screwthe carriage 104 when axially unloaded or subjected to a predeterminedaxial thrust travels at speeds which `are different in different zonesof the lead screw spaced longitudinally thereof. Thus when under suchconditions the differential rollers 113 drivingly cooperate with a firstzone 106 of the lead screw 105, the carriage 104 travels at a fasterspeed and when the differential rollers cooperate with Zone 109 of thelead screw the carriage 104 travels at a slower speed. Disposed betweenzones 106 and 109 of the lead screw is a transition zone 107 in whichthe speed of the carriage 104 is progressively slower as it travels fromzone 106 toward zone 109.

The carriage or cage 104 has an outer body with a first portion ofgenerally cup shape, such first portion having a base or transverse endportion 114 and a circular cylindrical sidewall portion 115. Within suchfirst portion f the housing, and abutting the end wall 114 thereof,there is a first radially slotted inner plate 116 which may be made, forexample, of tough strong plastic material, as indicated. A second, innerplate 117, which is similar to plate 116, is disposed adjacent theleft-hand end of such first portion of the housing, the plates 116 and117 being maintained in axially spaced parallel position by a spacersleeve 119 which is disposed therebetween and telescoped within thesidewall 115 of the first portion of the housing. The left-hand end ofthe sidewall 115 is provided with an inner -annular radial shoulder 120axially beyond which the sidewall is of thinner cross section, wherebyto provide an annular radially inwardly disposed seat. The outer edge ofplate 117 abuts shoulder 120 and is retained in the seat by an end plate121 which abuts the outer surface of plate 117 and is maintained inforcible gripping contact therewith by the incurled outer edge 122 ofthe thin walled portion of the sidewall 115. The end wall 114 and theplate 116, as one unit, and the plate 117 and cover plate 121, asanother unit, are each provided with a central passage 118 whichguidingly receives the outer surfaces of the lands of the lead screw105.

The lead screw 105 is shown as having a thread of modified Acme shape,such thread having a helical land 124, successive turns of the landbeing separated by a helical groove 125. The circular cylindrical rootsurfaces of such grooves are designated '126. Throughout the length ofthe lead screw the lands 124 have circular cylindrical outer surfaces ofa uniform or constant diameter. In the Zone 106, however, at which thecarriage 104 is traversed at a greater rate, the roots 126 of thehelical grooves 125 have a first smaller uniform radius A, and in zone109, at which the carriage 104 travels at a slower rate, the roots 126have `a second larger uniform radius C. In the zone 107 the roots 126 ofthe grooves 125 have a radius B which varies uniformly from the radius Aof zone 106 to the radius C of zone 109.

Within the housing of carriage 104 and spaced uniformly around the axisof the lead screw 105 there are a plurality (four shown) of theabove-mentioned differential rollers 113. The body of each of suchrollers, which are identical, is generally in the form of half of abarrel converging in a direction to the left. Each of the differentialrollers 113 has a thread of modified Acme shape thereon, such threadhaving a land 130 with an outer surface 131 of appreciable axial width.The height h of the land 130 is constant throughout the extents of bothportions of the thread on the roller 113. Between successive turns ofthe land 130 there is a helical groove 132, the depth of such groove,that is, the height h, of land 130, somewhat exceeding the height of theland 124 on the lead screw in the Zone 106 thereof. Intermediate itslength, each of rollers 113 is provided with an annular groove 135, thedifferential rollers being constantly urged radially inwardly toward thelead screw by a resilient belt or band 136, which may be in the form ofan O-ring, disposed in the grooves 135 in the rollers.

The differential rollers 113 'are guided for rocking movement in radialaxial planes through the lead screw as required to maintain drivingrelationship between the roots of the thread on the lead screw and theouter surfaces 131 of the lands on the differential rollers. Thus attheir right-hand ends the differential rollers are provided with stubshafts or pintles 137 having rounded'outer ends, shafts 137 beingslidably received within the `radial slots 139 in the guide plate 116.At their left-hand ends the differential rollers are provided withsimilar stub shafts 141 which are slidably received within radial slots142 in the left-hand guide plate 117. Theaxial distance between thecentral `rounded ends of stub shafts 137 and 149 is slightly less thanthe axial distance between the inner surface of housing portion 114 andthe inner surface 144 of the cover plate 121. Consequently, thedifferential rollers are free to rock, the upper one shown in FIG. 6rocking in a clockwise direction as the carriage 104 travels to theright along the lead screw 105 so that the differential rollers thendrivingly cooperate with the zone 107 of the lead screw. Following this,as the carriage enters the zone 109 the differential rollers rock in areverse direction'so as to return generally to the angular positionthereof shown in FIG. 6.

It will be apparent that when the carriage 104 is in a position tocooperate with zone 106 of the lead screw, the value of DL in theequation above-referred-to is small so that the ratio DL/DR is small andthe value of Lis large. When the carriage 104 cooperates with the Zone109 of the lead screw, however, the value of DL is larger so that thevalue of L is smaller. As above explained, the differential rollers areinclined at substantially the same angle relative to the axis of thelead screw when the carriage cooperates with either of zones 106 and 109of the lead screw. Consequently, no substantial variable enters theequation for the values of L at such zones. In the construction shown,there is no problem as to the maintenance of synchronism or phasing ofthe differential rollers, because they are self-adjusting. Should one ofthe differential rollers be temporarily overloaded, it will slow downits translational motion to relieve the excessive load thereon andthereby restore itself to synchronism with the other rollers.

It will be understoodthat the embodiment of variable gearing mechanismof FIGS. 6y and 7 is responsive to variations in the axial thrustimposed upon the carriage 104 as well as the above-discussed variationsin diameter of the root of the vhelical groove on the lead screw. Themounting of the differential rollers 113 is such that upon variations inend thrust upon the platen 101 and thus upon the carriage 104 thedifferential rollers each rocks about a transverse axis at right anglesto the longitudinal axis of the roller. Thus when a substantialretarding forceis irnposed upon platen 101 in the direction from rightto left, the differential rollers 113 rock about such transverse axis tobring `the smaller diametered zones of such rollers into drivingengagement with the flanks of the lands of the thread on the lead screw.The degree of rocking of the differential rollers, which is opposed bythe resilient belt or band'136, is proportional to the end thrustimposed upon the carriage; accordingly, the decrease in the speed oflinear travel of the carriage from its speed when it is axially unloadedis also proportional to such imposed end thrust or load on the platen. y

The fourth embodiment of gearing mechanism in accordance with theinvention is shown in FIGS. 8 and 9, where it is designated generally bythe reference character 145. A lead screw 146 having a modified Acmethread of uniform ldimensions has a helical 1and147, successive turns ofwhich are separated by a helical groove having a root 149. The leadscrew 46 is supported in suitable bearings (not shown) for rotationabout its axis while being held against axial movement. The lead screwmay be driven at end 150 thereof by power means not shown.

ra; Disposed about the lead screw for travellongitudinally thereofv is acage 'or carriage 151 having a housing in the formof a spring ring 152which is split by azradial opening 154. The ring'152 constantly thruststhe'rollers 113 forcibly radiallyfinwardlyrinto driving engagement 'withthe leadvscrew...Mountedupon. the ring 152 inrocking supports to bedescribed is a plurality of barrel shaped differential rollers 155 (fourshown) which are spaced uniformly, aboutthe axis of the lead screw.Each` ofthe rollers 155, which are symmetrical in'an axialdirection,have a thead of modified Acme shape with a land 156 of constant radialheight, successive turns of the land being separated by a groove l157having a root 159.

The rocker mounts for each of the rollers 155 are desi'gnated 160.Suchmou-nts have a`bo`dy`161 which is symmetricallin an axial direction,'thebody 161 having axially central radially outwardly extending parallelears 162. The Aouter ends of the ears 162 are connected to ring 151bya'pivot pinY 164 which extends generally as a'secant ofthe `outersurfaceY of the ring 152, the ears 160` being disposed within axiallyextending slots 165 through the ring. The relationshipbetween the4radial length of the ears 162 and the configuration of the body of-rollers 155 is such that the radially inner portions of the lands 156on the rollers which confront the lead screw as shown in FIG. 8 lie on aportion of a circle coaxial of the pivot pins 164.

At each o-f its ends the body 161 is provided with a generally radiallyinwardly projecting flange, that at the right in FIG. 8 being designated166 and that to the left 167. Each of the rollers 155 is provided at itslefthand end with a trunnion 169 'which is journalled in a bearing 170in flange 167. At its other end the body of the roller 155 is providedlwith an annular recess 171 which forms the outer race of a ballbearing. The balls 172 of such bearing are disposed in a circle withinsuch recess and cooperate with the axially inner conical end 175 of aset screw 174 which is threaded into the flange 166 of body 161, suchend 175 forming the inner race of the bearing.

The gearing mechanism is provided with means which maintain the rockingmounts 160 and the differential rollers 155 carried thereby in generallythe central position shown in FIG. 8 When the carriage 151 is subjectedto no appreciable endwise thrust. Such means, in the present embodiment,takes the form of a right-hand coil tension spring 176 and a similarleft-hand coil tension spring 177 for each of the rocker or rockingmounts 160, the axially inner ends or tangs of such springs beingconnected to a central cross pin 179 extending between the ears 162 ofthe respective rocker mount 160. The right-hand end of spring 176 isconnected to a cross pin 180 and the left-hand end of spring 177 isconnected to a cross pin 181, pins 180 and 181 being disposed adjacentthe axially outer ends of ring 152. When the carriage 151 is subjectedto appreciable endwise thrust in either direction, the rocker mounts 160pivot in the appropriate direction about the respective pivot pins 164.The effective driving diameter of the portion of the land 1-56 on thedifferential rollers which then engages the root 149 of the groove inthe lead screw is then decreased. Consequently, the value of DR in theabove equation is decreased, the value of the ratio DL/DR iscorrespondingly increased, and thus the value of L, that is, the rate oftravel of the carriage 151 at a given speed of rotation of the leadscrew 146, decreases as the axial thrust on the carriage increaseswithin limits.

Although a limited number of embodiments of the invention have beenillustrated in the accompanying drawings and described in the foregoingspecification, it is to be especially understood that various changes,such as in the relative dimensions of the parts, materials used, and thelike, as well as the suggested manner of use of the apparatus of theinvention, may be made therein without departing from the spirit andscope of the in- 10 vention, as will now be apparent to those skilled inthe art. f

I claim:

1. A variable differential gearing mechanism for converting rotarymotion to linear motion, comprising two members of which one is a leadscrew and the other of which is a differential roller, the two membersbeing disposed with parts thereof in side-by-side relationship, meansmounting the lead screw and the differential roller for rotation abouttheir respective axes, said last named means being so constructed andarranged as to -permit relative movement of said parts of thedifferential roller and the lead screw in a direction generally alongthe axis of the lead screw, the lead screw a-nd the vdifferential rollerhaving screw threads of the substantially same pitch thereon, each ofsaid screw threads having alternating helical lands and grooves,serially connected means including driving portions of the screw threadsfor drivingly connecting the two members for joint rotation, drivingmeans for rotating one of said members about its axis, and means forvarying the ratio of the effecti-ve diameters of the portions of the twomembers which are in driving relationship with each other, whereby uponrotation of the driven member the said parts of the two members travelgenerally axially vvith respect to each other at a speed which issubstantially a function of the instantaneous ratio between theeffective diameters of the portions of the screw threads on said twomembers which are in driving relationship with each other.

2. A mechanism as claimed in claim 1, wherein the two members aredisposed lwith said parts thereof in generally parallel relationship.

3. A mechanism as claimed in claim 1, wherein the threads on the twomembers are of opposite hand, and the two screw threads are drivinglyengaged in meshing relationship with the lands on one member disposedwithin the grooves on the other member.

4. A mechanism as claimed in claim 3, wherein the lead screw is directlydriven by the driving means, and the means for varying the ratio of theeffective diameters of the portions of the threads on the two memberswhich are in driving relationship with each other comprises meansresponsive to the end thrust imposed upon the differential roller forvarying the position of the axis of the differential roller relative tothe axis of the lead screw.

5. A mechanism as claimed in claim 4, wherein the means for varying theratio of the diameters of the zones of the two members which are indriving relationship with each other comprises means for varying thedistance between the axes of the lead screw and the differential roller.

6. A mechanism as claimed in claim 3, wherein the radially outersurfaces of the lands of the thread on said one member drivingly engagethe surfaces at the roots of the thread on the other member.

7. A mechanism as claimed in claim 6, wherein the said outer surfaces ofthe lands of the thread on said one member and the surfaces at the rootsof the thread on the other member are smooth and frictionally drivinglyengage each other, and comprising means for thrusting said memberstoward each other and said drivingly engaged surfaces thereof into tightengagement with each other.

8. A mechanism as claimed in claim 6, wherein the driving portion of thelead screw has, in that order, a first zone having a first substantiallyconstant effective diameter, a second zone having an effective diameterwhich varies at a substantially constant rate from said first diameter,and a third zone of substantially constant effective diameter extendingfrom the end of the second zone.

9. A mechanism as claimed in claim 6, wherein the differential roller isbarrel-shaped and has a large effective diameter at a zone intermediateits ends, such diameter decreasing from said zone of maximum diametertoward both ends of the diierential roller, and means for varying theinclination of the axis of the differential roller relative to the axisof the lead screw, whereby to vary the ratio between the effectivediameters of the portions of the threads on the two members which aredrivingly engaged with each other.

10. A mechanism as claimed in claim 3, wherein corresponding radiallyouter edges of the lands of the thread on one member drivingly engagethe corresponding sides of the lands on the other member at locationsspaced radially outwardly from the roots of the grooves of the thread onsaid other member.

11. A mechanism as claimed in claim 10, wherein the grooves on the othermember between the lands thereon are of greater axial'width throughout asubstantial portion of their radial depth than the width of the tips ofthe lands on the other member, and wherein the means for varying theratio of the effective diameters of the portions of the two memberswhich are in driving relationship with each other comprises means forvarying the distance between the axes of the lead screw and thedifferential roller.

12. A mechanism as claimed in claim 1, wherein the thread on one of saidmembers has a driving portion of constant effective diameter throughouta zone thereof of substantial axial length, `and the thread on the other'of said members has a driving portion of variable effective diameterthroughout a Zone thereof of substantial axial length, and wherein themeans :for varying the ratio of the effective diameters of the portionsof the threads on the two members which are in driving relationship witheach other comprises means for varying the inclination of the axes'ofthe lead screw and the differential roller relative to each other.

References Cited UNITED STATES 'PATENTS v Jenny 74-424.8

FRED C. MATTERN, JR., Primary Examiner. L. H. GERIN, AssistantExaminer,VV

